New and Easy Marking Method Tracks Bees Without Killing Them

This blue orchard bee (Osmia lignaria) was part of a new study to find a way to mark bees and track their movements. The “mark” is egg-white powder, shown here thickly covering the hairs on its thorax. This bee was photographed in a greenhouse foraging on a cover crop of blue tansy (Phacelia tanacetifolia). (Photo credit: Emily Slingerland, USDA)

By Leslie Mertz, Ph.D.

A wide range of bees play an important role in pollinating everything from wildflowers to agricultural crops. In fact, many farmers purchase bees to help get their fruits and vegetables pollinated. The question then becomes: Are those released bees actually staying in the farmer’s field to do the job, or are they flying off to places unknown?

Leslie Mertz, Ph.D.

Scientists have long sought a cheap and easy way to track released bees, but so far it’s been an uphill battle. Typically, scientists use a mark-and-recapture study, which means that bees are given an identifying mark of some sort, released into the field, and later collected in a trap or net to see how many of the captured bees are marked. That usually involves killing the bees to identify them to species and to check for the mark. Such mark-and-recapture studies don’t follow every single bee in a release, but they do provide an idea of whether it is the released bees—or other wild bees—that are actually doing the pollinating.

Current methods to mark bees include painting little dots or gluing tiny numbered tags onto the bodies of individual bees. While that works, it is grossly inefficient in pollinator studies where a typical release may number in the thousands of bees per acre.

Taking a Powder

The idea for the new technique grew from frustrations in research with blue orchard bees (Osmia lignaria), according to study co-author Natalie Boyle, Ph.D., a postdoctoral research entomologist at the U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) Pollinating Insects Research Unit. “Blue orchard bees are really good pollinators, and we’ve been using them in a number of trials, including for pears, sweet and tart cherries, and almonds. One of the biggest limitations is that we’re releasing hundreds of thousands of bees in our trials, so we never know how many of them are staying in the orchard or where they’re nesting,” she says.

Her frustrations progressed into a project during a 2016 visit to the USDA-ARS laboratory in Maricopa, Arizona. There, Boyle met up with USDA researchers Amber Tripodi, Ph.D., Scott Machtley, and James Hagler, Ph.D. Tripodi and colleague Jamie Strange, Ph.D., were similarly interested in marking bees, specifically certain bumble bees that are used to pollinate tomatoes and other crops in greenhouses. Machtley and Hagler, on the other hand, had considerable experience studying marking techniques for various insects. Before long, Hagler mentioned a good marking technique that has been used with many other insect species, and the group felt it might be a good option for bees.

To determine whether a bee has been marked with egg-white powder, entomologist Amber Tripodi, Ph.D., rinses it in buffer solution in a small vial. The bee is released, wet but otherwise unharmed, and the solution is then tested for the chemical signature of the powder. (Photo credit: Theresa Pitts-Singer, Ph.D.)

That technique relies on the application of powder made from dried egg whites (albumin): The powder sticks to the insect, and collected insects can later be checked for the presence of the powder with a straightforward test called an enzyme-linked immunosorbent assay (ELISA). It seemed feasible for tracking bees, Boyle says, because albumin powder is readily available at grocery stores and very inexpensive to test for.

To test whether it would work, the group had to figure out how to get the powder onto the bees with as little work as possible. That turned out to be relatively simple with the orchard bees and also with alfalfa leaf-cutting bees (Megachile rotundata), Boyle says. “Because it’s a dry powder, you can just roll the bee cocoons in that powder. As they chew their way out of the cocoon in the spring or summer to start foraging or looking for shelter, they self-mark themselves. For the bumble bee species Bombus bifarius, Amber had a colony in the lab and she used a little mesh sifter to sift a little bit of powder over the top of the colony and that was sufficient in marking them.”

Devising a No-Kill Method

The last hurdle in the method was to come up with a way to check bees for the mark without killing them. “The common method is to do lethal sampling, so we were pulling these individual pollinators out of the population in order to sample them for this protein. That was problematic, because the bees are not only providing this crucial pollination service, but they are retailing right now for about $1.50 per female,” Boyle explains. “We really needed to come up with a way to test for the powder without sacrificing these insects.”

They came up with an idea. Instead of killing the bees, perhaps they could rinse the bee in a saline buffer solution, and check the solution for a telltale sign of the albumin powder. To do it, they put a milliliter of the solution in a test tube, used forceps to carefully place a bee in the tube, shook the tube a bit to get the bee good and soaked, and then removed the wet but undamaged bee from the tube. An assay easily picked up the powder signature.

It worked perfectly in all three of the tested bee species: the blue orchard, leaf-cutting, and bumble bees. “I don’t think we could have asked for better results with this trial. Across the board, we got 100 percent detection of the mark and with no statistical difference at all in the survival of shaken versus not-shaken bees,” Boyle reports. “It seems like a pretty fool-proof method in our lab studies, so we’re really excited to take a final step this year and make sure everything works just as well in an open-field environment.”

With good results from the field work, this method could be useful beyond agricultural surveys, according to the paper’s authors. It could help investigate bee behavior in native ecosystems and provide insight into studies that explore the effect of urbanization, pesticides, or pathogens on bees, many populations of which are experiencing declines.

Overall, Boyle remarks, “This was a really fun project, and I couldn’t have imagined it going any more simply for us. It was very easy, very affordable, and very effective. That’s a triple-win right there.”